Effective tumor treatment is frequently thwarted by the lack of sensitivity of certain tumors to standard chemotherapeutic agents (intrinsic resistance) or by the ability of certain tumors to develop a lack of chemotherapeutic sensitivity during the course of treatment (acquired resistance). The cause of this phenomena has, at least in part, been demonstrated to result from the existance of an energy-dependent efflux pump which acts to remove the chemotherapeutic agent from the target cell.
The pump consists of P-glycoprotein found as a constitutent of cell membrane, and it has been suggested that the normal function of P-glycoprotein is to remove toxins from within the cell. This theory is supported by the observation that P-glycoprotein is found as a cell membrane constitutent in cells such as liver, kidney, colon, and jejunum. It has been suggested that P-glycoprotein in the cell membrane of such normal tissues could act to remove toxins or to assist in the transport of nutrients and solutes and in secreting a variety of protein and steroid substances. Natural presence of P-glycoprotein in tumor cells derived from these tissues as well as its presence in tumor cells derived from other tissue types could explain, at least in part, resistance of various tumors to therapy with standard chemotheraputic agents.
The use of therapeutic agents which inactivate the P-glycoprotein pump would be invaluable in the treatment of multidrug-resistant tumors. Quinidine and reserpine as well as the calcium channel blockers verapamil and diltiazem have been reported to reverse drug resistance in multidrug-resistant tumors. Such agents could function by, for example, interfering with transcription of the P-glycoprotein gene, blocking the drug binding site on the P-glycoprotein or by decoupling the energy dependent driving mechanism of the efflux pump.
Applicants have determined that certain pyridinyloxazole-2-ones having PKC inhibiting activity are useful in the treatment of multi-drug resistant tumors. The pyridinyloxazolone-2-ones of this invention act to reverse drug resistance and thereby allow standard chemotherapeutic agents to exhibit normal toxicity on tumors.